1. Field of the Invention
The invention relates to a method for operating an electrical device in which a very high current is typically generated when the device is turned on compared to the rated current. In particular, the invention relates to an analogue operated PTC heating device, an electrical pump, an electric motor, a magnetic coil or the like for a mobile device, in particular a vehicle.
2. Description of Related Art
A method is known for equipping mobile means, such as ships, rail vehicles, trucks, commercial vehicles, aircraft, spaceships and motor vehicles with heating devices which heat a flowing medium such as air. Such a device may be used in a vehicle interior heating system, an air heating system for window de-icing, a heating system for preheating the engine intake air, etc.
It is known to provide heating devices in the form of heating elements digitally operated via microprocessor control systems or without an electronic control system, for example on the basis of materials with a positive temperature coefficient of its resistance (PTC) or other heating elements, of metal for example, in which the elements have analogue operation and are switched on and off by means of mechanical relays. Heating elements may be provided in several stages which are switched on by a corresponding number of relays.
In such analogue operated PTC heating devices, a very high current, which may be equal to twice the rated current, flows when the device is switched on, i.e. in a connection phase whose length is of the order of a few seconds. This creates a current load for the power supply mains that is extraordinarily high, which may be considered disadvantageous.
Another problem in these types of devices is that there is no possibility of fault diagnosis, since switching takes place in stages, i.e. in unsteady conditions. Also, these systems involve the use of mechanical relays, causing switching noises. Service life is limited because of the switching cycles, and the operation incurs high costs because mechanical relays are expensive. The cost of wiring is also very high.
Heating systems without an electronic control system, i.e. analogue operated heating devices of the type described above are available on the market from a number of different suppliers.